The basic simplicity of ion-specific electrodes makes them attractive for the measurement of certain electrolytes in automated blood analysis instrumentation. Steele, et. al (Proc. San Diego Biomed. Sympos. 14: 469-478, 1975) have used a programmable calculator system to overcome many of the well-known problems of ion-specific electrodes -interferences, drifts, and liquid junction potential changes. They have also shown that such a calculatory system makes it possible to quickly convert the activities which ion-specific electrodes measure into the concentration units which are widely employed clinically for serum electrolytes. This conversion, is based on factors (serum water content, activity coefficients and ion binding by proteins) which may vary significantly in critically ill patients. In order to further establish the advantages and limitation of ion-specific electrodes for the determination of serum electrolytes of critically ill patients, a programmable calculator and data acquisition system with capabilities greater than that of Steele, et al. has been assembled, programmed and put into operation. The construction of a pH electrode and the design of a total CO2 measure device have been undertaken with the objectives of great simplicity and small sample size.